Method of passivating and encapsulating CdTe and CZT segmented detectors
A method of forming a passivation layer comprises contacting at least one surface of a wide band-gap semiconductor material with a passivating agent comprising an alkali hypochloride to form the passivation layer on said at least one surface. The passivation layer may be encapsulated with a layer of encapsulation material.
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The present invention relates generally to the field of semiconductor devices and particularly to radiation detectors.SUMMARY OF THE INVENTION
In one embodiment, a method of forming a passivation layer comprises contacting at least one surface of a wide band-gap semiconductor material with a passivating agent comprising an alkali hypochloride to form the passivation layer on said at least one surface.
In another embodiment, a method of passivating a monolithic, multichannel semiconductor radiation detector comprises (a) providing a cadmium telluride or cadmium zinc telluride substrate having a segmented anode electrode array on a first surface of the substrate and a cathode electrode on a second surface of the substrate and (b) forming a passivation layer at least between the anode electrodes of the anode electrode array by contacting the first surface of the substrate with a solution comprising sodium hypochloride.
In another embodiment, a radiation detector comprises (a) a semiconductor substrate having opposing front and rear surfaces, (b) a cathode electrode located on the front surface of the semiconductor substrate (c) a plurality of anode electrodes located on the rear surface of the semiconductor substrate (d) a passivation layer comprising an oxide of cadmium telluride or an oxide of cadmium zinc telluride located on the substrate between plurality of the anode electrodes, and (e) a layer of an encapsulation material disposed above the passivation layer, where the layer of encapsulation material comprises openings extending to the anode electrodes.
The following definitions are used herein:
Cathode electrode: the electrode on one major surface of the detector substrate where incident gamma rays or x-rays enter the detector, i.e. positioned towards the radiation source.
Anode electrodes: segmented electrode contacts located on the rear surface of the substrate, i.e. positioned away from the radiation source.
Interpixel or inter pixel: the region or gap separating pixel electrodes. For electrode configurations with non-pixellated discrete contact segments the term is equivalently applied to the gap between contact segments.
Radiation Detector Design
Radiation detectors can be configured in a variety of ways. A common configuration comprises a cathode electrode and a plurality of anode electrodes located on opposite sides of a semiconductor plate or substrate. Typically these monolithic multichannel radiation detectors have pixilated anode electrode arrays fabricated on a common semiconductor substrate by various deposition and lithography processes resulting in a gap between pixels, termed the interpixel gap or interpixel region. Each anode electrode pixel forms one of the plurality of detector channels.
In the preferred embodiments, the radiation detectors comprise a semiconductor material, such as a semiconductor material preferably comprising cadmium zinc telluride (CdZnTe or CZT) or CdTe. Although other types of semiconductor materials exemplified by lead iodide, thallium bromide, gallium arsenide or silicon may be used.
More preferred is Cd(1-x)ZnxTe (where x is less than or equal to 0.5), a wide band gap ternary II-VI compound semiconductor with unique electronic properties. This type of semiconductor is useful in gamma-ray and X-ray detectors which are used as spectrometers that operate at room temperature for radiation detection, spectroscopy and medical imaging applications.
The direct lithography fabrication process is described with reference to
In step 1, shown in
In an optional step 2 shown in
In step 3, as shown in
The resist coating is hardened in step 4 by baking for 10 minutes at 90° C. This step is done to drive excess solvent out of the resist. The tile is now prepared for lithography exposure.
In step 5, as shown in
In step 6 shown in
In step 7 the remaining resist pixel pattern 314 is baked for 20 minutes at 90° C. This step is done to harden the resist further.
In step 8, shown in
In step 9 shown in
The overall combination of depositing the metal layer over the entire substrate surface at once, direct photolithography and the etching process results in the improved device interpixel resistance and performance.
In step 10 shown in
Cathode Housing (Optional)
Optionally, an electrically conductive housing may be attached to the detector cathode by (a) providing a radiation detector comprising a semiconductor substrate having opposing front and rear surfaces, a cathode electrode located on the front surface of said semiconductor substrate and a plurality of anode electrodes on the rear surface of said semiconductor substrate, (b) providing a separately formed electrically conductive housing, and (c) attaching the housing to the cathode electrode such that the housing and the cathode electrode are in electrical contact.
A non-limiting example of a housing is depicted in
As shown in
The electrically conductive housing 425 shields the detector from background electromagnetic fields (or magnetic fields). Additionally, device electric fields are focused using this housing. The housing is also preferably transparent to X-ray or gamma-ray radiation. Further, the housing preferably exhibits little or no oxidation in ambient air, such as under normal operating conditions of the detector. As such, the housing is most preferably a thin structure and comprises a material transparent to radiation, substantially impervious to background electromagnetic fields and exhibits little or no oxidation at ambient conditions.
For example the housing may be between about 50 microns and 100 microns thick. In some cases a metal foil is sufficient as a housing.
Based on the parameters set forth above, one skilled in the art may chose from a host of materials for constructing the housing. In general, metals and metallic alloys are preferred. Any suitable metal which does not substantially oxidize in air may be used. A non-limiting example of suitable metallic alloys includes stainless steel, brass (such Ni/Ti coated brass), NiCo alloys, NiFe alloys, NiFeCo alloys, NiFeMo alloys or NiFeCuMo alloys. A class of metal alloys termed “Mu-metals” is most preferred. Mu-metals are a type of NiFe alloy, particularly effective at screening static or low frequency magnetic fields. In some cases, the aforementioned alloys may be doped with other alloying elements, mechanically pre-treated (e.g. cold worked, hot worked etc.), chemically surface-treated (e.g. surface coating for corrosion resistance) or any combination thereof.
In some cases, the housing 425 is shaped to conform to geometry of the detector, more specifically, to geometry of the cathode, to which it is secured. Therefore, one skilled in the art may contemplate various curved or angular housing shapes given the shape of the detector. In a non-limiting example, the housing is a rectangular or circular-cross-sectioned (e.g. cylinder) shape.
When the housing is constructed to extend over (partially or completely) at least one side of the substrate, said at least one side is spaced from said housing. This gap is either empty or filled with an insulating material.
The housing 425 is attached to the cathode electrode 200 such that an electrical conduction path exists between the two. In a preferred case, the housing and the cathode are attached via an electrically conductive material. Most preferably, an electrically conducive polymeric material, such as a conductive epoxy applied to the inner face or surface of the housing is used.
Edge coatings 420 suitable for certain embodiments of the present application are described in U.S. application Ser. No. 11/642,819, filed on Dec. 21, 2006 which is hereby incorporated by reference in its entirety.
Semiconductor Device Passivation
Semiconductor devices, such as segmented radiation detectors used in imaging applications (e.g. medical imaging), may be surface-passivated, for example to improve device reliability. Currently, passivation and encapsulation methods for single channel radiation detectors, are not practical for use with segmented multichannel monolithic radiation detectors for several reasons. For instance, single channel encapsulation procedures may not be compatible with the photolithography-based chemical fabrication processes needed for segmented detectors. Furthermore, certain passivation or encapsulation procedures may not be practical for large scale industrial production of segmented detectors. Even further, certain passivation techniques can only be applied on lower resistivity or narrow band-gap semiconductor materials. To that end, the present embodiments describe a method of forming a passivation layer on a semiconductor device. Such methods are practical for use with segmented radiation detectors as well as wide band-gap semiconductor materials. Specifically, in one aspect, the present embodiments describe a method of forming a passivation layer between anode pixels of a CZT or CdTe multichannel detector.
In one embodiment, a method of forming a passivation layer comprises contacting at least one surface of a wide band-gap semiconductor material with a passivating agent comprising an alkali hypochloride to form the passivation layer on the at least one surface of the material. Preferably the semiconductor material is incorporated into a radiation detector. For instance, the semiconductor material may be a semiconductor substrate having opposing front and rear surfaces where a cathode electrode is located on the front surface and a segmented anode electrode array is located on the rear surface of said semiconductor substrate, thus forming the detector. For example, and without any intent to limit the present embodiments,
In the context of the present embodiments, “passivation layer” refers to an oxide formed on the surface of the semiconductor material. As a non-limiting example, a passivation layer formed on a CdTe or CZT surface may comprise tellurium oxide. In certain cases, the surface of a semiconductor material to be passivated may already contain an oxide layer prior to contacting a passivating agent. For instance, the oxide layer may be a native or a thermal oxide layer. In such cases, the passivation layer formed according to the present embodiments is one resulting from contacting the semiconductor material surface (oxidized and/or unoxidized) with a passivating agent.
The passivating agent preferably comprises an alkali hypochloride which assists in forming a passivation layer on at least one surface of the semiconductor material. For example, the alkali hypochloride comprises an alkali metal, such as, but not limited to sodium, and hypochloride. More preferably, the alkali hypochloride reacts with cadmium telluride or cadmium zinc telluride to form an oxide. One or more types of oxides, such as a metal oxide or an oxide of a II-VI semiconductor material, may form depending on the composition of the semiconductor material surface. Without wishing to be bound by a particular theory, it is believed that the passivation layer comprises tellurium oxide. However, it is possible that the passivation layer may be a mixture of cadmium oxide, zinc oxide and/or tellurium oxide on CZT, or a mixture of tellurium oxide, cadmium oxide and/or cadmium telluride on CZT. One benefit, among others, is that the passivation oxides formed according to the present embodiments generally degrade less over time compared to oxides formed with other techniques.
The passivating agent may be applied to, or contacted with, the semiconductor material using a variety of techniques. For instance, the passivating agent may be in an aqueous or non-aqueous solution. The semiconductor material is partially or fully immersed into this solution for a period of time sufficient to form an oxide layer. In some cases, the step of applying the passivation agent may immediately follow or precede other surface treatment steps such as an etching step. As an alternative to immersion, the passivating agent may be sprayed or otherwise deposited onto the semiconductor material.
In a non-limiting example, the steps relating to contacting a semiconductor material with a passivating agent are as follows:
(1) prepare a solution of NaOCl in a clean beaker; for example a 1%-5% NaClO in DI water for 5-15 min.
(2) gently immerse a CZT tile(s) comprising pixellated anodes on one surface into the solution, with the anodes facing upwards;
(3) leave the tile(s) in the solution for at least 5 minutes, such as 5-15 minutes;
(4) remove the tiles and immerse in DI water for at least 2 minutes using gentle agitation;
(5) dry the tile(s) by patting with an absorbent fabric and blow dry with air.
The steps of forming the cathode electrode and passivation layer(s) can be performed in either order. In one embodiment, both anode and cathode electrodes of a radiation detector are formed prior to forming the passivation layer. In another embodiment, the passivation layer is formed prior to forming the cathode electrode of a radiation detector, but after forming the anode electrodes. In such cases, it may be desirable to avoid forming a passivation layer on the face of the substrate where the cathode will be subsequently formed by covering the cathode surface with a mask or by etching the passivation from the cathode surface.
The passivation layer can be formed on various parts of a semiconductor material, which is incorporated into a radiation detector device. In one embodiment, the passivation layer is formed only at the interpixel regions. In another embodiment, the passivation layer is formed only on the sidewalls of the semiconductor material. In yet another embodiment, the passivation layer is formed both at the inter-pixel regions and on the sidewalls. These embodiments are exemplified, without any intent to limit the same, in
Semiconductor Device Encapsulation
In one embodiment, the semiconductor devices comprise at least one a layer of an encapsulation material. Preferably, the encapsulation material encapsulates at least one passivation layer to protect the same. For example, a layer of encapsulation material can be applied onto to the passivation layer at the interpixel region to protect the passivation layer.
Examples of suitable encapsulation materials include, but are not limited to, aluminum oxide, aluminum nitride, aluminum oxynitride, parylene family of polymers (e.g. polyxylenes) and solder mask materials. Some solder mask materials may be chosen from a variety of photoimagable polymeric materials. Most preferably, the solder mask material comprises a UV-curable epoxy-based solder mask Solder masks are further described in U.S. application Ser. No. 11/642,819, filed on Dec. 21, 2006 which is hereby incorporated by reference in its entirely.
The solder mask material may be applied as a single continuous layer, several overlapping or butted layers or a combination thereof. The preferred methods of applying the solder mask are spray coating and dip coating.
A non-limiting example of the steps for forming a solder mask layer is as follows:
(1) Prepare an epoxy-based solder mask material with a suitable amount of thinner.
(2) Apply a layer of the solder mask material onto the anode face of a pixellated detector by spray-coating, spin-coating or dip-coating methods.
(3) Tack dry the layer at about 70-100° C., such as about 80-90° C., for a duration of about 15 minutes or more, such as about 20-30 minutes.
(4) UV-cure the solder mask layer at the interpixel area using photolithography, and develop the rest of the layer using a developer solution such as, for example 1-5% Na2CO3 solution.
(5) Cure the solder mask material at the interpixel region at about 120-150° C., such as at about 135-140° C., for a duration of about 0.5-2 hours, such as about 1-1.5 hours.
Semiconductor devices comprising an encapsulation layer are also exemplified in
Following photolithography and curing steps, a portion of the encapsulation layer 600 remains, which is shown as the cured encapsulation layer 620 in
Alternatively, as shown in
The encapsulation methods described herein provide various added benefits. For example, the encapsulation can ensure long term reliability of passivated devices. This may be critical for applications such as medical imaging and homeland security, which can not afford failure or loss of accuracy. Another benefit, is that the passivation and encapsulation techniques of the present embodiments are compatible with commercial packaging and general use in the art.
Although the foregoing refers to particular preferred embodiments, it will be understood that the present invention is not so limited. It will occur to those of ordinary skill in the art that various modifications may be made to the disclosed embodiments and that such modifications are intended to be within the scope of the present invention. All of the publications, patent applications and patents cited herein are incorporated herein by reference in their entirety.
1. A method of forming a passivation layer comprising:
- contacting at least one surface of a wide band-gap semiconductor material with a passivating agent comprising an alkali hypochloride to form the passivation layer on said at least one surface.
2. The method of claim 1, wherein semiconductor material comprises a semiconductor substrate.
3. The method of claim 2, wherein:
- the semiconductor substrate comprises a substrate of a multi-channel monolithic radiation detector;
- the semiconductor substrate has opposing front and rear surfaces;
- a cathode electrode is located on the front surface of said semiconductor substrate; and
- a segmented anode electrode array is located on the rear surface of said semiconductor substrate.
4. The method of claim 1, wherein the wide band-gap semiconductor material comprises a II-VI semiconductor substrate.
5. The method of claim 4, wherein the alkali hypochloride comprises NaOCl.
6. The method of claim 5, wherein the semiconductor material comprises cadmium telluride or cadmium zinc telluride.
7. The method of claim 4, wherein the passivation layer comprises an oxide of the II-VI semiconductor material.
8. The method of claim 3, wherein the passivation layer is formed at least between anode electrodes of the segmented anode electrode array.
9. The method of claim 3, wherein the passivation layer is formed at least on sidewalls of the substrate.
10. The method of claim 1, wherein the step of contacting comprises immersing the semiconductor material in a solution comprising the passivating agent.
11. A method of passivating a monolithic, multichannel semiconductor radiation detector comprising:
- providing a cadmium telluride or cadmium zinc telluride substrate having a segmented anode electrode array on a first surface of the substrate and a cathode electrode on a second surface of the substrate; and
- forming a passivation layer at least between the anode electrodes of the anode electrode array by contacting the first surface of the substrate with a solution comprising sodium hypochloride.
12. The method of claim 11, wherein the semiconductor substrate comprises cadmium zinc telluride.
13. The method of claim 11, wherein the step of contacting lasts between about 5 and about 60 minutes.
14. The method of claim 11, wherein a passivation layer is further formed on sidewalls of the substrate.
15. The method of claim 11, wherein the cathode electrode is formed before the passivation layer.
16. The method of claim 11, wherein the cathode electrode is formed after the passivation layer.
17. The method of claim 11, wherein a thickness of the passivation layer in between the anode electrodes is less than or equal to a height of the anode electrodes.
18. The method of claim 11, wherein the passivation layer comprises an oxide of cadmium telluride or an oxide of cadmium zinc telluride.
19. The method of claim 11, further comprising drying said first surface of the substrate.
20. A device formed according to the method of claim 1.
21. A device formed according to the method of claim 11.
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Filed: Aug 8, 2008
Date of Patent: Jun 7, 2011
Patent Publication Number: 20100032579
Assignee: Redlen Technologies, Inc. (Saanichton, BC)
Inventors: Henry Chen (Victoria), Pinghe Lu (Victoria), Salah Awadalla (Victoria)
Primary Examiner: Jarrett J Stark
Attorney: The Marbury Law Group, PLLC
Application Number: 12/188,501
International Classification: H01L 21/00 (20060101);